Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 33.4.11 NO X reduction during startup/shutdownSee also Section 3.4.10 “Addition of H 2 O 2 to the last absorption stage”.DescriptionThe process for producing nitric acid is unstable at startup and shutdown. At startup, NO xemissions are higher (varying from 600 – 2000 ppm/1230 – 4100 mg NO x /m 3 ) for the first 10 –45 minutes, giving an extra emission of 100 – 1000 kg NO x /year. Shutdown results are in thesame range as NO x concentrations (600 – 2000 ppm/1230 – 4100 mg NO x /m 3 ) for 10 –30 minutes, giving a maximum extra emission of 500 kg NO x /year.During normal operation, the percentage NO 2 in the NO x varies between 50 – 75 %. Especiallyduring startup, the NO 2 emission is larger than the NO emission (70 % NO 2 , 30 % NO),colouring the stack gas reddish-brown or yellow. The visual appearance of this stack gas oftenresults in complaints from people in the locality. This is a local problem and therefore will notbe described in detail.Various techniques are available to reduce NO X emissions during startup and shutdown, butinvestments are often high compared to the amount of NO x reduction achievable. Extra NO xemissions due to startup and shutdown are less than 1 % of the overall NO x emissions during ayear. The measures could be considered to reduce the visual appearance of a fume.Potential end-of-pipe measures during startup include:Heating the tail gas. A furnace, steam heater or separate burner could be used to heat the tailgas. Operated between 180 and 450 °C, at the optimum for SCR/NSCR, this measure willreduce NO x at an earlier stage and minimise the startup period.Installing a low temperature SCR. Installing an SCR facility with a large temperature rangewill allow NO X reduction at lower temperatures (from 180 °C and higher) and as a result, downtimesfor the SCR will reduce.Installing a scrubber. Excessive emissions at startup can be minimised by the use of caustic,sodium carbonate or ammonia alkali scrubber.Installation of a tower with a dry adsorbent material. NO x reduction at startup can be achievedby using a dry adsorbent, e.g. silica gel or molecular sieves.The above procedures are less suitable during shutdowns, as pressure and temperature quicklyreduce.Achieved environmental benefitsHeating the tail gas. Exact figures are not known, but a NO x reduction of at least 70 % duringpart of the startup can be expected.Installing a low temperature SCR. Exact figures are not known. Despite the lower temperature(from 180 °C and higher), some extra emissions during startup will take place, but for a shorterperiod of time. During the period that the SCR facility operates, a NO x reduction of 80 – 95 %can be obtained.Installing a scrubber. Exact figures are not known, but a NO X reduction of at least 70 % duringstartup can be expected.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 137